No matter how young or old you are, blackouts should always be evaluated by a physician. Unfortunately your description of your blackouts are not detailed enough to narrow down the possible causes, but I will try to give you a few possibilities.
One possibility is that you are experiencing orthostatic syncope. This is where your blood pressure drops upon standing up from a seated or lying position causing blood to fall from your head down towards your feet. The end result is lightheadedness, darkened vision, and passing out completely after standing up. There are many causes of this (too many to go through hear). Another possibility is that you are having seizures. People who have seizures can blackout for short periods of time. If the seizure is not witnessed, then there is no way to know that it happened.
I suggest that you schedule an appointment with your primary care physician
. He or she can perform tests of your blood pressure and heart rate while standing and sitting which will help tell if you have a problem with orthostasis. Your doctor may also order a tilt table test which can help tell what's causing these blackouts. No matter what the results, you may need an echocardiogram and a brain MRI to make sure there are no other problems going on. You may warrant referral to a cardiologist
depending on what is found on these tests. Good luck.